Breakfast has become the final frontier in the struggle over health, convenience and profit in the U.S. diet.

Starved for growth at other mealtimes, companies like Yum Brands Inc., Burger King Worldwide Inc. and Kellogg Co. are battling to change how Americans start their day. But even as consumers back away from cereal to experiment with more protein-rich foods, their habits remain otherwise entrenched: most Americans still eat...